The present invention relates to a method of holding a substrate and a substrate holding system to hold a substrate securely during a production process for treating the substrate, such as semiconductor device, while it is being cooled.
Among substrate treating apparatuses for production of semiconductor devices, there are a lot of substrate treating apparatuses requiring the cooling substrates, such as a plasma treatment apparatus, a sputtering apparatus, a dry etching apparatus, a CVD (chemical vapor deposition) apparatus and a high energy ion implantation apparatus. Since the treating environment in these apparatuses is generally a vacuum, it is difficult to cool a substrate by bringing it into contact with a cooling surface, as in atmospheric pressure, because of the decrease in thermal conductivity which occurs in a vacuum. Although there is abundant literature concerning thermal conductivity in a vacuum (rarefied gas), the amount of heat transferred by contact is small because of the small real contact area when surfaces come into contact with each other. Especially in heat transfer between a substrate and a cooling surface, it is difficult to strongly press the substrate against the cooling surface since there is a possibility to damage the substrate. Therefore, various ideas, such as placing a soft elastomer on the surface contacting the substrate, have been proposed. However, in recent years, it has been more conventional for a gas to be introduced between the substrate and a cooling surface to cool the substrate using the gas as a coolant, when the heat load in the substrate increases or a requirement to cool the substrate to lower the temperature thereof arises.
There are various types of gas cooled substrate holding systems. They can be roughly categorized as follows: (1) a gas cooling type system where the back surface of a substrate and a cooling surface contact each other and a gas is introduced into the gap between the surfaces formed by the surface roughness, and (2) a gas cooling type system where the back surface of a substrate and a cooling surface do not contact each other and a gas is introduced into the gap between the surfaces.
The prior art gas cooling type systems belonging to the former category (1) are described in, for example, Japanese Patent Publication No. 2-27778 (1990), Japanese Patent Application Laid-Open No. 62-274625 (1987), Japanese Patent Application Laid-Open No. 1-251375 (1989), Japanese Patent Application Laid-Open No. 3-154334 (1991) and Japanese Utility Model Application Laid-Open No. 4-8439 (1992). And, the prior art gas cooling type systems belonging to the latter category (2) are described in, for example, Japanese Patent Application Laid-Open No. 63-102319 (1988), Japanese Patent Application Laid-Open No. 2-312223 (1990), Japanese Patent Application Laid-Open No. 3-174719 (1991). Further, there is another type of system, described in Japanese Patent Application Laid-Open No. 2-30128 (1990), where, before introducing a cooling gas, the back surface of a substrate and a cooling surface are brought into contact with each other, but during cooling the substrate is pushed up due to gas pressure caused by introducing the cooling gas and does not contact the cooling surface.
In these cooling systems, providing that a certain cooling gas is used, the cooling capacity (magnitude of transferred heat) with the cooling gas depends on the pressure of the gas and the distance between the back surface of a substrate and the cooling surface (gap in the back surface of the substrate). FIG. 8 schematically shows the characteristic of thermal conductivity in a low pressure situation. When the pressure of the cooling gas is low, the amount of transferred heat is proportional to the pressure of the cooling gas and independent of the magnitude of the gap between both of the surfaces. When the pressure of the cooling gas is higher than the pressure PO, where the mean free path of the cooling gas nearly coincides with the gap, the amount of transferred heat becomes constant and independent of the gas pressure. The pressure of the cooling gas in the type of system in category (1) described above is generally in the region where the heat transfer is proportional to pressure, and the pressure of the cooling gas in the type of system in category (2) described above is generally in the region where the heat transfer is independent of pressure.
Characteristics and problems in various methods of cooling a substrate will be described below.
First, a description will be made on the case where cooling is performed under a condition that a substrate contacts a cooling surface. The cooling methods belonging to this type are disclosed in Japanese Patent Publication No. 2-27778 (1990), Japanese Patent Application Laid-Open No. 62-274625 (1987), Japanese Patent Application Laid-Open No. 1-251375 (1989), Japanese Patent Application Laid-Open No. 3-154334 (1991) and Japanese Utility Model Application Laid-Open No. 4-8439 (1992). In the cooling method of this type, although the substrate and the cooling surface contact each other, only the most protruding portions on the cooling surface contact the substrate when it is observed in detail. The indented portions on the cooling surface and on the substrate do not contact each other, and the gaps are approximately 10 xcexcm to 50 xcexcm, although this depends on the surface roughness. In a case where a cooling gas is introduced in the gap, the pressure is generally several Torrs, which is in a region nearly equal to the mean free path. Therefore, a sufficient cooling efficiency can be obtained by properly setting the pressure as shown in FIG. 8.
However, when the cooling gas is supplied from a specified single portion, as shown in the figure in Japanese Patent Publication No. 2-27778 (1990), the pressure is highest in the cooling gas supplying portion and decreases as it goes toward the peripheral portion of the substrate. Since the cooling efficiency has a pressure dependence as shown in FIG. 8, there arises a disadvantage that uniformity of the temperature distribution is deteriorated due to the non-uniformity of the cooling efficiency. If there is no gas leakage, that is, no gas flow, the pressure distribution does not occur and the temperature distribution becomes uniform. However, in order to achieve this, the peripheral portion of the substrate needs to be shielded. This is described in Japanese Patent Application Laid-Open No. 62-274625 (1987) or in Japanese Utility Model Application Laid-Open No. 2-135140. Further, the method in which cooling gas is supplied from plural portions to make the pressure distribution on the back of the substrate uniform is described in Japanese Patent Application Laid-Open No. 1-251735 (1989) or in Japanese Patent Application Laid-Open No. 4-61325 (1992). In any case, in these cooling methods, since the back surface of the substrate and the cooling surface contact each other in a large area, there is a disadvantage in that a lot of foreign substances become attached to the back surface of the substrate when contacting the cooling surface. Further, in order to prevent the cooling gas from leaking through the peripheral portion of the substrate using a shielding material, a load for the shielding needs to be applied. Therefore, a way of tightly fixing the substrate in some manner is required.
Description will be made below of a cooling method where a substrate and a cooling surface do not contact each other and a cooling gas is supplied into the gap. The prior art method is described in Japanese Patent Application Laid-Open No. 3-174719 (1991) or in Japanese Patent Application Laid-Open No. 4-6270 (1992), in which a substrate is mechanically fixed to a cooling surface from the top surface or the side surface of the substrate. Since the substrate, in these examples, is mechanically fixed, there is a disadvantage that foreign substances are apt to be produced at the fixing portion. In the methods described in Japanese Patent Application Laid-Open No. 63-102319 (1958) and in Japanese Patent Application Laid-Open No. 2-30128 (1990), a substrate is not fixed specially, but is held by the weight of the substrate itself. In this case, in order that the leakage of the cooling gas is not increased too much or the substrate is not caused to float up, the pressure of the cooling gas has to be limited to a low level. This causes a disadvantage in that the cooling efficiency is decreased.
Electrostatic adhesion is a known method of fixing a substrate electrically. An example where a substrate is fixed to a cooling surface with this method and projections are provided on the periphery of the substrate is described in Japanese Patent Application Laid-Open No. 62-208647 (1987). A substrate contacts a cooling surface only at a plurality of projections provided in separate spaced relation on the outer periphery and inner periphery of the substrate, which is described in the Japanese Patent Application Laid-Open No. 62-208647 (1987). And, this publication indicates that cooling gas easily leaks and that the adhering force is unstable. Further, in order to improve this method, it is effective if the outer periphery is not projected and the projections are provided only on the inner peripheral portions, and further, if the projections in the inner periphery are provided in the central portion, instead of in separate spaced relation. In this case, the gap between the substrate and the cooling surface becomes non-uniform over the surface of the substrate, which causes a non-uniform pressure distribution on the back surface of the substrate. When the gap between the back surface of the substrate and the cooling surface varies from one position to another, the ratio of the mean free path of the cooling gas and the gap has an uneven distribution over the surface of the substrate. Therefore, a disadvantage arises in that the temperature distribution is apt to become large due to the difference in cooling efficiency, as can be understood from FIG. 8, even if the pressure distribution is not so large. In the electrostatic adhering method described in this example, there are provided positive and negative electrodes on the cooling portion to which a direct current high voltage is applied to produce an electrostatic adhering force. In the electrostatic adhering method of this type, there may arise a disadvantage in that, when a substrate is treated in a plasma, the electric charge on the surface of substrate produced by irradiated ions or electrons is apt to be non-uniform, and so a current flows on the surface of substrate to damage the substrate.
Each of the conventional technologies, as described above, has the main objective of cooling a substrate efficiently. However, with an increase in integration of semiconductor devices in recent years, it is required to decrease the amount of small foreign substances, such as particles or dust and heavy metal impurities, to less than the allowable limit in the past. The same can be said for foreign substances attached on the back surface of a substrate. When the amount of foreign substances attached on the back surface of a substrate is large, there arises a disadvantage in the next process in that the foreign substances on the back surface are attached to the top surface of an adjacent substrate, or are removed first from the substrate and attached to another substrate. Therefore, decreasing the amount of foreign substances is an important problem for stabilizing the semiconductor production process or improving the yield. Attaching of foreign substances on the back surface of a substrate occurs by contacting the back surface of the substrate to another member. Therefore, a lot of foreign substances are attached to a substrate by contacting a cooling surface for the substrate.
Further, the prior art does not refer to the consideration of substrate size. Although it is mentioned that the influence upon the process is lessened by leaking cooling gas into the treating chamber, with the adhering force being as small as possible, the relation between the adhering force and the cooling gas pressure is not mentioned.
A conventional substrate holding system in a substrate etching apparatus generally employs a method in which a substrate is pressed along its periphery with hooks, as described in Japanese Patent Application Laid-Open No. 2-148837 (1990) or Japanese Patent Application Laid-Open No. 2-267271 (1990). When there is such a member contacting the surface of the substrate, problems arise in that the contact portions in the substrate are obstructed by the etching, the contacting member itself being also etched to some extent together with the substrate. As a result, the foreign substance sources, such as reaction products, are attached to the contacting member and the contacting member may be damaged, which may lead to production of foreign substances.
On the other hand, in a substrate holding method in which a substrate is held using electrostatic force (hereinafter, referred to as xe2x80x9celectrostatic adheringxe2x80x9d), as described in, for example, Japanese Patent Application Laid-Open No. 2-135753 (1990), a substrate is placed on an electrostatic adhering portion made of a dielectric material and a high voltage is applied to hold the substrate with an electrostatic adhering force. In this case, there is no special member to press the substrate in the periphery of the substrate. Therefore, the problem of the possibility of producing foreign substances as described in the above example is solved. However, the positional relationship between the substrate and the electrostatic adhering member is such that the substrate is placed in the uppermost position (substrate etching space side) and a step is formed in the electrostatic member such that the electrostatic adhering member comes to be placed below the substrate. When such a step exists, gas flow during etching a substrate changes abruptly at the periphery of the substrate to cause a non-uniform etching in the substrate in some cases.
An object of the present invention is to provide a method of holding a substrate and a substrate holding system in which the amount of foreign substances on the back surface can be decreased, and only a small amount of foreign substances may be transferred from a mounting table to a substrate.
Another object of the present invention is to provide a method of holding a substrate and a substrate holding system where the deformation in a large diameter substrate can be suppressed, and the cooling efficiency for the substrate can be kept sufficiently high.
A further object of the present invention is to provide a method of holding a substrate and a substrate holding system in which damage to the substrate caused during treating the substrate can be prevented.
A still further object of the present invention is to provide a method of holding a substrate and a substrate holding system in which a cooling gas can be quickly dispersed over the back surface of a substrate when the cooling gas is introduced, after the substrate is electrostatically attracted, and substrate temperature control suitable for high productivity can be performed.
Another object of the present invention is to improve the product yield of substrate etching and the availability of the substrate etching apparatus providing a substrate holding system which is subjected to a reduced amount of foreign substances as described above and is capable of performing uniform etching.
According to the present invention, there is provided a ring-shaped leakage-proof surface having a smooth surface on the specimen table corresponding to the periphery of the substrate, a plurality of contact holding portions which bear against and support the substrate on the specimen table between the corresponding position to the periphery of the substrate and the corresponding position to the center of the substrate, and electrostatic attraction means for generating an electrostatic attraction force to attract the substrate toward the ring-shaped leakage-proof surface and the contact holding portions.
In order to decrease the amount of foreign substances which may adhere on a substrate, it is effective to decrease the contact area between a cooling surface and a substrate. However, the distance between the cooling surface and the back surface of the substrate needs to be kept at a distance so as not to decrease the cooling efficiency of a cooling gas. In order to realize this, a small high step is provided on the cooling surface, such that the back surface of the substrate and the cooling surface do not contact each other whether the cooling gas is introduced or not. Although the cooling surface and the back surface of the substrate contact each other at protruding portions provided on the step portion of the cooling surface, the area of the contact portions needs to be limited so as to be as small as necessary. In the present invention, therefore, an electrostatic attraction function is given to the cooling surface to attract the substrate to the protruding portions of the cooling surface.
Prevention of leakage of the cooling gas has to be considered. According to the present invention, this can be attained by providing a ring-shaped protruding portion having a smooth surface, that is, leakage-proof surface, on the cooling surface corresponding to the peripheral portion of a substrate, and fixing the back surface of the substrate to the cooling surface with electrostatic adhesion to prevent the cooling gas from leaking.
According to the present invention, the following effects are obtained. One of the effects is a solution for the problem concerning the transportation of foreign substances in a pusher portion relating to handling of the substrate. The pusher provided inside or through a mounting table contacts other members and cannot avoid a foreign substance source. In the present invention, the excess cooling gas flows toward the opposite side of the mounting table through the hole. Since the foreign substances produced are carried in the opposite direction to the substrate, the amount of foreign substances attached to the substrate is decreased.
Another effect of the present invention is that a cover is provided in the back surface of the mounting table to protect the mechanism in the back surface of the mounting table from being contaminated by reaction products over a long time of use as much as possible. Since complex mechanisms, such as the coolant supplying system and the vertical driving mechanism for the mounting table, are usually constructed in the back of the mounting table, it is troublesome when the reaction products produced by etching attach to these parts. In order to avoid this, according to the present invention, a cover is provided in the back surface of the mounting table such that the excess cooling gas flows into the inside of the cover, the pressure inside the cover being kept higher than the pressure in the treatment chamber during treating to suppress the reaction products from entering the treatment chamber, which protects the mechanism in the back surface of the mounting table from contamination by reaction products over a long time of use.
The prevention of damage to the substrate can be attained by connecting the electric circuit for electrostatic adhesion from the substrate side to a grounded part, such as the vacuum chamber, through the plasma to minimize the electric potential over the surface of the substrate.
According to the present invention, a substrate contacts a cooling surface at a ring-shaped leakage-proof surface and at contact holding portions positioned inwardly of the ring-shaped leakage-proof surface. However, since the back surface of the substrate does not contact the cooling surface in most of the remaining part of the area, attaching of foreign substances caused by contact can be prevented. Although the cooling efficiency for the substrate cooling is decreased a little compared to when the substrate contacts the cooling surface under the same pressure of the cooling gas, a sufficient cooling efficiency can be obtained by forming a step on the cooling surface smaller than approximately 100 times the mean free path of the cooling gas. The gap between the back surface of the substrate and the cooling surface is large in comparison to that in the conventional cooling method, where the substrate and cooling surface contact each other over the whole surface. Therefore, the conductance between both surfaces is large so that supplying and exhausting of cooling gas are easily performed. That is, the time to supply and exhaust the cooling gas is short, and so the time for treating a substrate can be shortened. Further, there is a function that the conductance at the contact portion of the periphery of the substrate and the cooling surface is very small in comparison to the non-contact portion of the inner portion of the substrate (in the molecular flow region, the conductance is proportional to the square of gap), the pressure difference across the non-contact portion being small, that is, the cooling efficiency being uniform.
When the substrate temperature is controlled using a cooling gas as a coolant, the pressure of the cooling gas is required to be higher than 2 Torrs. And, the higher the pressure is, the higher the efficiency of heat transfer becomes. On the other hand, the electrostatic adhering force largely depends on the temperature of the substrate being controlled. In a typical production line today, the temperature is approximately xe2x88x9260xc2x0 C. to +100xc2x0 C., and an adhering force of 40 to 100 gf/cm2 is stably obtained under an applied voltage in general of 300 to 1000V. Concerning the pressure control of the cooling gas, it is difficult to control the pressure precisely, since the pressure largely changes depending on the time constant of the gas supplying system or the relationship between the relative roughnesses of the contact surfaces of the substrate and the mounting table. Therefore, the target of the pressure control may be, for example, 10 Torrsxc2x15 Torrs.
When the outer periphery of the substrate is fixed by adhesion with the conventional method and a gas is filled in the back of the surface with the pressure of 10 Torrs, the substrate is deformed by 0.1 to 0.25 mm. This magnitude of deformation degrades the work accuracy of substrate etching as well as lessens the heat transfer efficiency of the cooling gas. To solve this problem, adhering portions are additionally provided on the center side of a substrate, for example, one ring-shaped adhering portion for a 6xe2x80x3 substrate, two ring-shaped adhering portions for an 8xe2x80x3 substrate, in addition to the adhering portion on the periphery of the substrate. Therewith, the deformation can be prevented.
It is well known that when a substrate contacts another member, foreign substances are certainly attached to the contact point. From this point of view, it is clearly preferable that the electrostatic adhering surface is small. However, taking the pressure control level and adhering force into consideration as described above, it is suitable in the up-to-date technical level that the adhering area is less than approximately half of the total area of the substrate. This is because, when the electrostatic adhering force is 40 gf/cm2 and the adhering area is half of the total area, the total adhering force for an 8xe2x80x3 substrate becomes approximately 6280 gf, and the separating force with the cooling gas of 15 Torrs is approximately 6100 gf.
Further, by providing a pusher for substrate transportation in a hole penetrating the back surface of the mounting table, the excess cooling gas serves as a carrier gas for carrying foreign substances produced at the pusher portion to prevent the foreign substances from attaching to the substrate. In addition to this, the excess cooling gas is introduced into the inside of a cover on the back surface of the mounting table and makes the pressure inside the cover higher than the pressure in the treating chamber to prevent contamination of and attaching of reaction products to the mechanisms on the back of the mounting table.
In order to prevent abnormal discharge from occurring when the high frequency voltage is applied to the substrate to generate a bias voltage for etching the substrate, a high frequency voltage applying portion and a standard electric potential portion are insulated with an electrical insulating material so as not to face each other directly. In addition to the above measures, a pin for transporting the substrate is provided and is constructed so as to be electrically conductive. Since the electrostatic adhering force due to a remaining charge can be instantaneously dissipated by removing the charge accumulated in the substrate by causing the pin to contact the substrate when the substrate is transported, the substrate is not lifted with unnecessary force.
Since the flow passage to conduct the coolant for controlling the temperature of the substrate is formed by diffusion welding or soldering in such a structure that the portion forming the flow passage is completely jointed, no seal is required when a through hole is provided in any place except the flow passage. Therefore, a temperature detector or a detector for detecting the presence or absence of substrate can be easily provided.
In order to make the gas flow on the surface of a substrate uniformly, a gas flow controlling member (hereinafter referred to as a xe2x80x9csusceptorxe2x80x9d) is provided in the outer peripheral portion of the substrate. The surface of the susceptor is at a higher level than that of the substrate so that the gas flow does not abruptly change direction at the periphery of the substrate. The surface of the susceptor facing the periphery of the substrate is formed normal to the surface of the substrate to restrict movement in the lateral direction or sliding of the substrate. Further, there are some cases in which the reaction products produced by substrate etching or the plasma flows into the gap between the cover member facing the back surface of the substrate in the peripheral portion and the back surface of the substrate to cause foreign substances to attach on the back surface of the substrate. This phenomena is prevented by the distance between the back surface of the substrate and the cover, member.
As described above, since contact between a foreign substance source and a substrate is eliminated as far as possible, the transfer of foreign substances to the substrate can be decreased. Further, since the gas flow is made uniform, the uniformity in the substrate etching over the surface can be improved. Since the detector for measuring substrate temperature and the detector for detecting the presence or absence of a substrate can be easily installed by modification of the structure and the manufacturing method of the substrate holding system, the reliability and the operability of the apparatus can be improved.